The present invention relates to fixation systems for use in stabilizing and immobilizing spinal segments, particularly in the cervical spine.
Bone fixation devices are useful for promoting the proper healing of injured or damaged spinal motion segments caused by trauma, tumor growth, degenerative disc disease or other spinal pathologies that may necessitate permanent immobilization. These fixation devices are typically used to immobilize the injured/damaged bone or motion segments to ensure the proper growth of new osseous tissue between the damaged segments.
One type of fixation system utilizes an osteosynthesis plate, more commonly referred to as a bone plate that can be used to immobilize adjacent skeletal parts such as vertebral bones. Typically, the fixation plate is a rigid metal or polymeric plate positioned to span bones or bone segments that require immobilization with respect to one another. The plate is fastened to the respective bones using anchors, such as bone screws, so that the plate remains in contact with the bones and fixes them in a desired position. Cervical plates, for instance, can be useful in providing the mechanical support necessary to keep vertebral bodies of the cervical spine in proper position, and in bridging a weakened or diseased area such as when a disc, vertebral body or spinal fragment has been removed. These cervical plates usually include a rigid bone plate having a plurality of screw openings in the form of holes or slots that allow for freedom of screw movement. The bone plate is placed against the damaged vertebral bodies and bone screws are used to secure the bone plate to the spine, usually with the bone screws being driven into the vertebral bodies. Cervical bone plates are typically placed anteriorly, although posterior or transverse fixation is also known.
Because the cervical spine is routinely subject to mechanical loads which cycle during movement, one of the primary concerns is the risk of screw pullout. This is of particular concern in the cervical region because of the critical vessels that abut the anterior surfaces of the cervical spine. Screw pullout often occurs when the cylindrical portion of the bone which surrounds the inserted screw fails. A bone screw which is implanted perpendicular to the plate is particularly weak because the region of the bone which must fail for pullout to occur is only as large as the outer diameter of the screw threads. It has been found that for pullout to occur for a pair of screws which are angled relative to each other and to the plate, the amount of bone which must fail increases substantially as compared to pairs of screws which are implanted in parallel along the axis that the loading force is applied. It has, therefore, been one goal of those in the art to provide a cervical screw-plate assembly that permits the screws to be entered into the vertebral body at angles other than 90 degrees.
As mentioned above, a great concern with screws being implanted in the anterior portion of the cervical spine is that there are important internal tissue structures which may be damaged by a dislocated screw. In the cervical spine, the esophagus is located directly in front of the anterior surface of the vertebral body, and therefore, in potential contact with an implanted cervical plate. Because screw pullout represents one of the largest risks of esophageal perforation, it has been a further goal object of those in the art to produce a cervical screw-plate design that prevents the screw from separating from the plate, even if the bone holding the screw fails.
One typical screw-plate design provides angled holes for insertion of the bone screw. This typical design, as represented by the Orion® Anterior Cervical Plate System of Sofamor Danek, further includes an additional threaded hole disposed between pairs of bone screw holes so that a corresponding set screw may be inserted to lock the bone screws to the plate. Although the Orion® system achieved certain advantages over prior cervical screw plate assemblies, one drawback is that a given plate can accommodate only one screw angular orientation per hole. This is undesirable, in that physicians often must inspect the vertebral bodies during the implantation procedure before making the decision as to which screw-in angle is the ideal. While providing a variety of plates having different angle bone screw holes is possible, the complexity and expense of providing a full spectrum of plates available in the operating room for the surgeon to choose from is undesirable.
In order to address the concerns of these screw-plate systems, other systems have been developed that permit polyaxial coupling of the screw to the plate, whereby a single plate is compatible with a wide range of screw-in angles. In typical systems of this type, the head of the bone screw is spherical to match a spherical surface on the holes formed in the cervical plate. The bone screw can thus be oriented at a wide range of angles relative to the plate. In other systems, the bone screw is seated within an insert that is mounted within the screw holes in the plate, wherein the insert permits the variable angle placement of the screw. In both systems, a separate component, such as a set screw, is required to lock the assembly in position and prevent back-out of the bone screw from the plate.
There remains a need for an orthopedic screw plate assembly which provides an effective mechanism for engaging a bone screw to the plate at any desired angular orientation. It is desirable that the assembly require as few parts as possible to simplify the surgeon's task during implantation into the cervical spine.